STAFF PHOTO BY DONALD STOUTPumps, at left, have been moved onto a test site where engineers will determine how much water the London Avenue Canal can safely hold.

By Sheila GrissettEast Jefferson bureau

Using crack meters, prisms, piezometers and a slew of other sensitive instruments, the Army Corps of Engineers turned a small slice of the London Avenue Canal into a $4 million laboratory Friday to determine how much water it can safely hold.

The two-week test calls for gradually pumping water into a dammed-off section of the canal's east side and monitoring the pressure it exerts on the levee, floodwall and bottom of the channel, all the while guarding against a structural failure.

"We've got enough instrumentation that we would never even begin to approach failure," said Maj. Nick Nazarko, the corps officer in charge of safety in the 5700 block of Warrington Drive.

Although long on detail and complexity, the goal of the exercise is simple.

The corps wants to raise the designated "safe water elevation" above four feet, a level so low that it has on occasion interrupted drainage even during routine rains. But the level cannot come up without convincing evidence that the repaired canal, which broke at two spots during Hurricane Katrina, can handle additional water.

To that end, the corps and its advisers have custom-designed a test using a 35-by-150-foot cofferdam. The three-sided dam was built of steel sheeting adjacent to the east floodwall about 1,000 feet south of Robert E. Lee Boulevard, thought to be the canal's weakest spot.

When the experiment begins, water within the dam will be at the same height as the canal and Lake Pontchartrain. Over a period of days, water will be added in six-inch increments.

At each step, even the slightest movement of wall, levee or the canal's sandy subfloor will be measured by an assemblage of instruments strategically placed in the canal, on the floodwall, along the levee's crest, banks and toes and even at the tips of the sheet pile walls driven some 40 feet or so into the canal floor.

"These instruments are so sensitive that they'll pick up movement before you can see it with your eyes," Nazarko said.

All movement will be recorded as it occurs and fed into a bank of computers clustered in a work trailer sitting beside the floorwall. Inside, technical personnel with the URS Corp., the project's instrumentation subcontractor, will montior the data on a 24-hour basis.

After each half-foot of water is pumped into the cofferdam, it will be held until Nazarko, using a detailed decision matrix and in concert with other project team leaders, decides that it is safe to add more.

"We're looking for things to stabilize, and when the technical folks say that has happened and with concurrence of the team, I'll make to call to go to the next level," Nazarko said.

It also will be Nazarko who calls a halt to the process if instrument readings suggest a pause is in order.

The test will help set water elevations that are safe against two potential failure modes:

-- Formation of a crack between the floodwall and earthen levee, as happened during Katrina on the London Avenue and 17th Street canals.
-- Water penetrating the marshy canal bottom to and soaking deeper into the sand beneath it. Canal water can then exert such pressure on the sand that the force blows out a levee section.

Corps representatives said that if Hurricane Dean threatens the area before the test is complete, they have a contingency plan to secure the site and eliminate sections of the coffer dam if warranted.

Even in the absence of a hurricane, the plan has built in several safeguards in the event of a water spill, a potential that Nazarko rates as "very, very, very unlikely."

A sluice gate within a wall of the cofferdam can be raised to empty excess water from the contained site, and on-site pumps are also able to drain water from the dam. Nazarko said a specially trained team will stand by if needed to close new floodgates to isolate the canal and test site from Lake Pontchartrain. On the levee's landside, a temporary dam is in place to contain any water that might escape. And observers are scattered about the canal to watch for any indicators of trouble.

But Nazarko said the first line of defense against crisis remains keen instrumentation and data monitoring.

Dozens of complex devices pepper the site, including crack meters, tilt meters, pressure-measuring piezometers and inclinometers, which gauge the angle between the earth's magnetic field and a horizontal plane.

There is even a robotic machine programmed to shoot thousands of beams of invisible light at more than 40 survey prisms. The return bounce of light provides yet another method of tracking the movement of walls or levee sections.

Team members spent Friday determining baseline measurements and tending to last-minute details before the first few inches of water can be added, which isn't expected to happen until today or Sunday.

Once all the data is collected, it will take several more weeks to analyze it and determine whether it provides a sufficient empirical basis on which to raise the four-foot safe water level throughout the canal.

Sheila Grissett can be reached at sgrissett@timespicayune.com or (504) 717-7700.